This invention relates to a demodulating circuit for deriving a first signal and a second signal from a signal comprising a resultant signal of the first and second signals, a carrier-suppressed AM signal obtained by AM-modulating a carrier with a difference signal between the first and second signals, and a pilot signal, and in particular, to a demodulating circuit adaptable to demodulate a stereo signal including a Subsidiary Communication Authorization (SCA) signal.
In a known stereo broadcasting system for broadcasting both of the right and left side audio signals (which will be referred as R and L, respectively) by a broadcast wave, a main carrier is FM-modulated with a modulating signal comprising a main channel signal of (R+L), a sub-channel signal of a carrier-suppressed AM signal which is obtained by AM-modulating a sub-carrier (usually, 38 KHz) with a difference signal (R-L), and a pilot signal (usually, 19 KHz), and, then, is broadcasted. In the reception, the modulating signal is obtained by FM-demodulation of the received signal, from which the left side signal L and the right side signal R are separately derived at a demodulating circuit of the composite signal or, so called, a multiplex demodulator.
A known method for deriving the both signals R and L from the FM-demodulated signal is to apply a composite signal comprising at least the main channel and sub-channel signals to a switching circuit comprising two gates which are alternatingly switched on or off by a switching signal having the same frequency as the sub-carrier. The R and L signals are obtained at the outputs of the two gates, respectively. The switching signal is made from the pilot signal by the use of a phase-locked oscillator or a frequency doubler. The composite signal may include the pilot signal, which is removed at a de-emphasis circuit provided at a stage following the switching circuit. The pilot signal may be, in another way, removed before the switching circuit by a filter or the like. In that case, the composite signal does not include the pilot signal.
In some cases, an SCA signal is included in the stereo broadcast wave. The SCA signal is made by FM-modulating a secondary sub-carrier (usually, 67 KHz) with a certain signal, and is combined with the stereo modulating signal. The main carrier is FM-modulated with the resultant signal and is broadcasted. The SCA signal serves as a disturbance in the reception of the stereo broadcast.
In the demodulation of the above described switching type, beats between the SCA signal and harmonics of the switching signal are generated and are present at the demodulated output.
In order to remove such a disadvantage, a, so-called, SCA filter has been used for removing the SCA signal from the FM-demodulated signal. But, the use of the SCA filter badly affects the amplitude and the phase of the sub-channel signal, and, in practical use, cannot completely prevent the generation of such beats.
A demodulator circuit which resolves such a disadvantage is proposed in a Japanese patent application No. 37888/'71 which was laid open under a publication No. 3409/'73.
FIG. 1 shows a typical demodulator circuit proposed in the prior Japanese patent application.
Referring to FIG. 1, two outputs from the switching circuit SW are respectively applied to two inputs of a differential amplifier DA comprising transistors Tr.sub.1 and Tr.sub.2. Emitters of both transistors are commonly connected to a constant current circuit comprising a transistor Tr.sub.3. The composite signal, which may or may not include the pilot signal, is applied to the base of the transistor Tr.sub.3.
According to the circuit arrangement, even if the SCA signal accompanies the stereo signal, the beats between the SCA signal and the harmonics of the switching signal are suppressed at the differential amplifier DA because the beats present in the two outputs from the switching circuit SW are in-phase to one another. Furthermore, the composite signal is introduced to the differential amplifier DA through the transistor Tr.sub.3, so that the signals R and L are obtained from output terminals Out.sub.R and Out.sub.L of the differential amplifier DA, respectively.
If the beat signals are represented by E.sub.B, the two outputs from the switching circuit SW are represented as (R+E.sub.B +e.sub.1) and (L+E.sub.B +e.sub.2), respectively, where e.sub.1 and e.sub.2 represent high frequency components.
Assuming that the composite signal is represented by (R+L)+(R-L).multidot.sin .omega.t+e.sub.s +p.multidot.sin .omega./2t (where, R-L).multidot.sin .omega.t is the sub-channel signal, e.sub.s is the SCA signal, and p.multidot.sin .omega./2t is the pilot signal), the outputs e.sub.R and e.sub.L of the differential amplifier DA are represented by following equations: EQU e.sub.R =(R+E.sub.B)-(L+E.sub.B)+(R+L) EQU =(r-l)+(r+l) EQU =2r EQU e.sub.L =(L+E.sub.B)-(R+E.sub.B)+(R+L) EQU =(l-r)+(r+l) EQU =2l
namely, audio signals R and L are separately obtained from output terminals Out.sub.R and Out.sub.L of the differential amplifier DA. It should be noted that the high frequency components e.sub.1, e.sub.2, (R-L).multidot.sin .omega.t, e.sub.s, and p.multidot.sin .omega./2t are removed by de-emphasis circuits comprising a resistor R.sub.1 and a capacitor C.sub.1, and a resistor R.sub.2 and a capacitor C.sub.2.
The proposed circuit in the prior Japanese patent application enables to effectively remove the affection of SCA signal without the use of any SCA filter.
But, in the proposed circuit, the transistor Tr.sub.3 amplifies the composite signal. And the amplified main channel signal (R+L) is applied to the emitters of the transistors Tr.sub.1 and Tr.sub.2. Therefore, the main channel signal (R+L) is considerably distorted because of the low impedances of the transistors Tr.sub.1 and Tr.sub.2 which are loads of the transistor Tr.sub.3.
Furthermore, it is hard to balance the R and L output signals because of the characteristic difference of the transistors Tr.sub.1 and Tr.sub.2.
The use of diodes D.sub.1 -D.sub.4 for the switching circuit causes the generation of a click noise at a time of the exchange between the monophonic broadcast and the stereo broadcast, because of the characteristic difference of diodes D.sub.1 -D.sub.4.